Various methods are known for imparting catalytic activity to carbonaceous chars by treatment of such chars with nitrogen-containing compounds. In some cases a high-temperature char such as charcoal or activated carbon is heated at temperatures above 700.degree. C. in the presence of a nitrogen-containing compound such as ammonia or an amine. In other treatment processes activation of the char with steam and exposure to the nitrogen-containing compound occur simultaneously. Other variations of the process involve incorporating the nitrogen-containing compound directly into the raw material used to produce the char. In another method, the nitrogen-containing compound is introduced after low-temperature carbonization and oxidation of a nitrogen-poor char feedstock but before high temperature exposure and condensation of the carbon structure. This method produces the highest functional utility with the least economic and environmental costs. For the purposes of the following discussion, all catalytic carbons produced by such processes will be hereinafter referred to as "nitrogen-treated carbons."
Nitrogen-treated carbons have the ability to function as catalysts per se without the addition of metal impregnants. They have utility in a number of applications, particularly hydrogen sulfide removal from gas streams containing oxygen and water vapor. However, during use, the carbon catalysts prepared by such processes become spent or deactivated, requiring the carbon catalyst to be replaced with a fresh supply of catalyst material. Currently, no methods exist for the regeneration of nitrogen-treated carbons used for hydrogen sulfide removal from gas streams containing oxygen and water. Moreover, it is not known how the chemistry of hydrogen sulfide removal occurs on nitrogen-treated carbons or how this chemistry may affect the catalyst sites created by the nitrogen treatment.
It is known that conventionally activated carbons and activated carbons impregnated with salts such as caustic soda or caustic potash remove hydrogen sulfide from gas streams containing oxygen and water by oxidation of the hydrogen sulfide to elemental sulfur, i.e., EQU 2H.sub.2 S+O.sub.2 .fwdarw.2S+2H.sub.2 O (1).
Small amounts of sulfuric acid have also been observed as a reaction product in certain cases; however, this phenomenon is considered to be a minor and unwanted side reaction the origins of which have been generally attributed to the presence of iron or other ash impurities. The elemental sulfur created by reaction (1) deposits in the pore structure of the carbon until the operative moiety responsible for the catalysis is occluded.
Methods for regenerating conventional activated carbons deactivated in this manner have relied upon solvents or upon thermal treatments to remove the accumulated elemental sulfur. The solvents used in such methods are invariably hazardous, expensive, or inconvenient to use. Carbon disulfide, the solvent of choice in many applications, is highly volatile and flammable as well as expensive and highly toxic. Water, the cheapest, safest, and most convenient solvent, cannot be used for regeneration since elemental sulfur is insoluble in water.
Thermal techniques may also be used to restore hydrogen sulfide capacity in conventional carbons deactivated by exposure to hydrogen sulfide. Elemental sulfur sublimes at temperatures above 445.degree. C. and may, therefore, be removed from the carbon surface by direct heating or by hot gases such as nitrogen or steam. Where steam or other oxidizing or reducing agents are present, various other sulfur compounds will also be produced. Since these methods generate significant quantities of sulfurous vapors, post-treatment facilities such as acid scrubbers or Claus plants are generally required. Additionally, these methods are energy intensive and require materials of construction which must withstand both high temperatures and corrosive gases. As a result, the utility of thermal treatment methods is limited.
The caustic-impregnated carbons are usually regenerated by contact with concentrated caustic solutions in which elemental sulfur is highly soluble. Since concentrated solutions of sodium or potassium hydroxide are corrosive and toxic, these methods are also hazardous, expensive, and inconvenient to use. Thermal treatment methods are generally not a suitable option for caustic-impregnated carbons since the impregnant catalyzes gasification of the carbon structure at high temperatures and creates aerosols which corrode common materials of construction. Where transition metals are used to impregnate the carbons, comparable problems are encountered. Furthermore, the use of an oxidizing agent is generally required to restore the functioning of transition metal-impregnated carbon catalyst. Such treatments also create significant amounts of sulfurous off-gases which require extensive post-treatment.
Accordingly, it is the object of the present invention to provide a method for regenerating with water a spent nitrogen-treated carbon used for hydrogen sulfide removal.